CPT International 03/2015

MATERIALS
risk that, in case of a high yield ratio,
failure of the component will occur instantaneously
once the yield point has
been exceeded.
Figure 11 shows the yield ratios of
the tested tensile samples of grades EN-
GJS-500-14 and EN-GJS-400-15.
Like in the case of the static performance
values, as described above, porosity
also has an effect on the yield
ratio. The yield ratio increases with increasing
porosity. However, the rise is
only significant in the case of porosity
levels 2 and 3. Consequently, the overload
allowance of ductile cast iron decreases
with increasing porosity.
A highly critical aspect is the fact
that this increase in yield ratio is the
result of decreasing tensile strength
and not of increasing yield strength
with the tensile strength remaining at
the same level.
Cyclic performance values
The S/N curves (50 % probability of
fracture) of the reference specimens
and of the specimens of porosity level
1 are plotted in Figure 12. The fatigue
limit for alloy EN-GJS-500-14 is
above that of EN-GJS-400-15 for both
specimen types (see S/N curves 1 and 2
versus S/N curves 3 and 4). The higher
Si content of alloy EN-GJS-500-14 has
a positive effect on the cyclic performance
values, similar to its effect on
the static performance values.
Figure 12 also shows that the fatigue
limit of the samples of porosity
level 1 is about 20 MPa or 8 –10 %
lower than that of the reference samples.
This holds true for both alloy EN-
GJS-400-15 and alloy EN-GJS-500-14.
Figure 13 compares the results from
the horizontal tests (selected samples
of alloys EN-GJS-500-14 and EN-
GJS-400-15 subjected to a constant
stress of 230 MPa) with the results from
the defect analyses by means of Volume
Graphics in the fracture region
of the same samples (see figure 7). It is
obvious that the fatigue limit decreases
along with a growing porosity volume
(3-D) and a growing projection
area (2-D).
The influence of the area fraction of
porosity (2-D) on the lifetime is also
Figure 14: Effect of porosity on the lifetime of the tested specimen (porosity
determined by metallographic 2-D microsection analysis)
a
b
Figure 15: SEM analysis of fracture surface in an EN-GJS-500-14 reference specimen:
a) panoramic view, b) residual fracture area, c) area of the fatigue crack
/ crack initiation
c
18 Casting Plant & Technology 3/2015